An effective prophylactic HIV-1 vaccine is needed to eradicate the HIV/AIDS pandemic but designing such a vaccine is a challenge. Despite many advances in vaccine technology and approaches to generate both humoral and cellular immune responses, major phase-II and -III vaccine trials against HIV/AIDS have resulted in only moderate successes. The modest achievement of the phase-III RV144 prime-boost trial in Thailand re-emphasized the importance of generating robust humoral and cellular responses against HIV. While antibody-directed approaches are being pursued by some groups, others are attempting to develop vaccines targeting cell-mediated immunity, since evidence show CTLs to be important for the control of HIV replication. Phase-I and -IIa multi-epitope vaccine trials have already been conducted with vaccine immunogens consisting of known CTL epitopes conserved across HIV subtypes, but have so far fallen short of inducing robust and consistent anti-HIV CTL responses. Thus, a need remains in the art for an effective vaccine against HIV.
Domestic cats are subject to infection by several retroviruses, including feline leukemia virus (FeLV), feline sarcoma virus (FeSV), endogenous type C oncoronavirus (RD-114), and feline syncytia-forming virus (FeSFV). Of these, FeLV is the most significant pathogen, causing diverse symptoms including lymphoreticular and myeloid neoplasms, anemias, immune-mediated disorders, and an immunodeficiency syndrome that is similar to human acquired immune deficiency syndrome (AIDS). Recently, a particular replication-defective FeLV mutant, designated FeLV-AIDS, has been more particularly associated with immunosuppressive properties.
The discovery of feline T-lymphotropic lentivirus (now designated as feline immunodeficiency virus, FIV) was first reported in Pedersen et al. (1987). Characteristics of FIV have been reported in Yamamoto et al. (1988a); Yamamoto et al. (1988b); and Ackley et al. (1990). Seroepidemiologic data have shown that infection by FIV is indigenous to domestic and wild felines throughout the world. A wide variety of symptoms are associated with infection by FIV, including abortion, alopecia, anemia, conjunctivitis, chronic rhinitis, enteritis, gingivitis, hematochezia, neurologic abnormalities, periodontitis, and seborrheic dermatitis. The immunologic hallmark of domestic cats infected with FIV is a chronic and progressive depletion of feline CD4+ peripheral blood lymphocytes, a reduction in the CD4:CD8 cell ratio and, in some cases, an increase in CD8-bearing lymphocytes.
Cloning and sequence analysis of FIV has been reported in Olmsted et al. (1989a); Olmsted et al. (1989b); and Talbott et al. (1989). Hosie and Jarrett (1990) described the serological response of cats infected with FIV. FIV virus subtypes can be classified according to immunotype based on the level of cross-neutralizing antibodies elicited by each strain (Murphy and Kingsbury, 1990). Recently, viruses have been classified into subtypes according to genotype based on nucleotide sequence homology. Although HIV and FIV subtyping is based on genotype (Sodora et al., 1994; Rigby et al., 1993; and Louwagie et al., 1993), little is known about the correlation between the genotype and immunotype of subtypes. FIV viral isolates have been classified into five FIV subtypes: A, B, C, D, and E (Kakinuma et al., 1995; Yamamoto et al., 2007; Yamamoto et al., 2010). Infectious isolates and infectious molecular clones have been described for all FIV subtypes except for subtypes C and E (Sodora et al., 1994). Subtype C FIV has originally been identified from cellular DNA of cats from Canada (Sodora et al., 1994; Rigby et al., 1993; Kakinuma et al., 1995). Examples of FIV strains identified in the art include (subtype of the strain is shown in parenthesis) Petaluma (A), Dixon (A), UK8 (A), Dutch113 (A), Dutchl9K (A), UK2 (A), SwissZ2 (A), Sendai-1 (A), USCAzepy01A (A), USCAhnky11A (A), USCAtt-10A (A), USCAlemy01 (A), USCAsam-01A (A), PPR (A), FranceWo, Netherlands, Bangston (A/B), Aomori-1 (B), Aomori-2 (B), USILbrny03B (B), TM2 (B), Sendai-2 (B), USCK1gri02B (B), Yokohama (B), USMAsboy03B (B), USTXmtex03B (B), USMCg1wd03B (B), CABCpbar03C (C), CABCpbar07C (C), CABCpady02C (C), Shizuoka (D), Fukuoka (D), LP3 (E), LP20 (E), and LP24 (E).
The commercial release of an effective HIV-1 vaccine is not imminent even after completion of four major phase IIB-III vaccine trials against HIV/AIDS (Saunders et al. (2012)). Our limited understanding about the mechanisms of vaccine protection (Plotkin (2008)) and the identity of the protective viral epitopes (Mothe et al. (2011); Koff (2010)) further hampers the development of an effective vaccine. Initial studies focused on antibody-based vaccine designs with an emphasis on generating broadly virus neutralizing antibodies (bNAbs) (Stamatatos (2012)). However, two phase-III vaccine trials using envelope (Env) immunogens failed (Flynn et al. (2005); Pitisuttithum et al. (2006)). Subsequent focus was placed on the T-cell-based vaccines that generate protective cell-mediated immunity (CMI) against global HIV-1 isolates (Buchbinder et al. (2008)). The CMI responses, essential for an effective vaccine, most likely include cytotoxic T lymphocyte (CTL) activities that specifically target HIV-1 infected cells (Ogg et al. (1998); Walker et al. (1988); Belyakov et al. (2012)). Unlike NAb epitopes which reside exclusively on the Env proteins, the selection of specific vaccine epitopes for the development of T-cell-based vaccines is more difficult to achieve. A vast number of CTL-associated epitopes can be found to span the whole length of most HIV proteins (Los Alamos National Laboratory (LANL) database, hiv-web.lanl.gov/content/immunology/maps/maps.html) (Llano et al. (2009)). The goal to develop T-cell-based vaccines is challenged by the capacity of the virus to evade antiviral immunity through mutation(s) for resistance (Li et al. (2011); Leslie et al. (2004)).
A recent phase III trial consisting of priming with a gag-pr-gp41-gp120 canarypox vectored vaccine and boosting with Env gp120 induced both humoral immunity and CMI and conferred a modest overall efficacy (Rerks-Ngarm et al. (2009)). However, phase I and II vaccine trials consisting of cross-subtype conserved CTL-associated peptide epitopes have shown minimal CMI responses (Sanou et al. (2012a); Hanke et al. (2007); Salmon-Ceron et al. (2010)). Therefore, a thorough selection of potent anti-HIV T cell-associated epitopes, which are conserved among HIV-1 subtypes and do not mutate without negatively affecting viral fitness (Troyer et al. (2009); Goulder et al. (2008); Rolland et al. (2007)), would be valuable for an effective HIV-1 vaccine. One approach is to select conserved, non-mutable CTL epitopes on essential viral structural proteins or enzymes that also persist on the older subgenuses of the lentivirinae which have survived evolutionary pressure (Yamamoto et al. (2010)). Such an approach was successfully used in the development of the initial smallpox vaccines (Jenner (1798)). In line with this strategy, the recognition of conserved epitopes on other lentivirus species has been made by the PBMC from HIV-1 positive (HIV+) humans (Balla-Jhagjhoorsingh et al. (1999)), HIV-2 vaccinated and SIV-challenged non-human primates (Walther-Jallow et al. (2001)), and HIV-1 p24-vaccinated and FIV-challenged cats (Abbott et al. (2011); Coleman et al. (2005)).
The viral enzyme, reverse transcriptase (RT), is one of the most conserved viral proteins by possessing the lowest entropy value among the HIV-1 proteins from various subtypes (Yusim et al. (2002)) and contains many CTL-associated epitopes (Walker et al. (1988)). The RT proteins of HIV-1 and FIV also share the highest degree of identity in their amino acid (aa) sequences (Yamamoto et al. (2010)). The current studies were undertaken to identify the conserved CTL-associated epitopes on FIV and HIV-1 RT proteins which are recognized by the PBMC and T cells from HIV+ subjects. The major objective of such studies is to identify evolutionarily-conserved CMI epitopes that may be more resistant to mutation, and thus useful in the development of an effective, T-cell-based HIV-1 vaccine.